Technical Field
The present disclosure relates to the field of fluids dynamics, and in particular, to the control of instabilities related to a boundary layer. Some exemplary applications of the present disclosure can relate to the field of aerodynamics, and in particular, to the control of turbulent boundary layer flows as known in the aeronautics industry.
Description of the Prior Art
A critical issue in the aeronautics industry is the energy consumed to deal with turbulent boundary layer flows. This energy consumption occurs not only on the wings of an aircraft but also on the fuselage (e.g. see reference [1] and FIG. 1). The force generated by such turbulent boundary layer flows increases the fuel consumed by the aircraft in each phase of a flight, from takeoff to landing. Thus, in an exemplary case of an aircraft, affecting such turbulent boundary layer flows (turbulent flow, turbulence) can lead to significant reduction in fuel consumption. As known to a person skilled in the art, other applications associated with turbulence modification at a surface of a vehicle subjected to a fluid can include, for example, boundary layer state, such as transition advance and transition delay, augmented or reduced heat transfer, augmented or reduced scalar mixing, separation control and skin friction drag modification.
In the field of hydrodynamics, usage of compliant coatings on surfaces of a ship is one technique that has been explored to reduce (turbulent) drag due to the turbulent boundary layer flows as shown, for example, by the work of Kramer (see reference [5]). The analysis of Bushnell et al. (see reference [2]) and the experimental studies from Choi et al. (see reference [3]) and from Gad-el-Hak (see reference [4]) have led to the conclusion that an appropriate compliant coating is an efficient passive technique to reduce the turbulent drag by delaying laminar to turbulence transition. However, it is known in the art that it is difficult to measure small changes in drag with high accuracy. The state of the art is that there remains a question as to the viability of passive compliant walls for the control of turbulence (due to the multi-scale nature of a transitional boundary layer), whereas their usefulness in delaying the onset of turbulence has been convincingly demonstrated. It is noted that an ideal passive coating to control the boundary layer transition (e.g. turbulence) is dependent on flow conditions, which of course can change.